IntroductionActive Galactic Nuclei (AGN) are the nuclei of Galaxy which show energetic phenomena fromextremely compact volumes that can’t clearly and directly be attributed to stars. One dis-tinctive characteristic of AGN is the fact that their energy flux varies along the whole elec-tromagnetic spectrum, spanning a wide range of time scales from year to hours. Indeed, fluxvariability is often-used criterion for AGN detection (e.g. Mushotzky 2004). The combinationof high luminosity and short variability time scale implies that the power of AGN is producedby phenomena more efficient in terms of energy release per unit mass than ordinary stellar pro-cesses (Fabian 1979). This basic argument leads to the hypothesis that massive black holes arepresent in the core of AGN. Accretion of matter onto a black hole or extraction of its rotationalenergy can in fact yield high radiative efficiencies (Rees et al. 1982, Rees 1984). Within thewidely accepted canonical model, i.e. super massive black hole (SMBH) + accretion-disk (AD)+ dusty torus (DT) + relativistic jets, different regions of the active nucleus are thought tocontribute to the power emitted at different frequencies along the spectral energy distribution(SED).Active Galactic Nuclei (AGN) classified into two largest subclasses are Seyfert galaxies andQuasars. Seyfert galaxies are the subclass of AGN hosts defined as a galaxies with bright,star-like nuclei that have relatively broad emission lines covering a wide range of ionization, inwhich host galaxy are clearly detectable. It become obvious that not all spectra from Seyfertgalaxies look same, so they are sub-classified into two types according to standard spectralclassification of Khachikian & Weedman (1974). They differ by presence or absence of broadbase on the permitted emission lines. Type 1 Seyfert have both broad-line region (BLR) andnarrow-line region (NLR) present in their spectra while Type 2 Seyfert have only narrow-lineregion (NLR) present in their spectra. Narrow-line Seyfert 1 (NLSy 1) galaxies are special classof lower-luminosity AGN, defined by Osterbrock & Pogge (1985). They show width of narrowoptical balmer emission line FWHM(H?)< 2000 km s ?1 (Osterbrock et al. 1985, Goodrichet al. 1989), flux ratio of OIII ?5007 /H?< 3 (Shuder-Osterbrock et al. 1981), (however,exceptions are possible if they have strong Fe VII and Fe X lines) , strong permitted opti-cal/UV Feii emission lines (Boroson-Green et al. 1992, Grupe et al. 1999), steep soft X-rayspectra (Wang et al. 1996, Boller et al. 1996), rapid X-ray flux variability (Leighly et al.1999, Komossa-Meerschweinchen et al. 2000), and rapid optical flux variability (Miller et al.2000). They also show the radio-loud/radio-quite dichotomy, however, the radio-loud fractionof NLSy1 galaxies are about 7 per cent (Zhou et al. 2003), smaller than the fraction of 15per cent known in the population of quasars (Urry and Padovani 1995). Among the general3AGN classes, many sub-classes such as NLSy1 galaxies display very peculiar properties. Theinterpretation of such peculiarity remain incomplete due to lack of multi-wavelength observa-tion and/or due to the small number statistics. With the advent of large spectroscopic surveysuch as SDSS their sample sizes have now been increased substantially, and need to be analysesystematically using the recently launched multi-wavelength space mission like ASTROSAT inconjunction to the ground based observation. This form the main motivational of this thesisby focusing mainly on NLSy1 subclass of AGN, on key problems as listed in brief below.A brief review of work already done in the fieldOver the last decades with the advent of the large survey such as SDSS/BOSS, there havebeen rather extensive research to fairly understood the questions including AGN geome-try/structure, the nature of the energy and the continuum source, emission line regions, andthe factors that produce an AGN in some galaxies and not in others. The main goal of ourstudy is to carry out the systematic investigation of the less explored NLSy1 galaxies in thecontext of widely accepted AGN unification model, where various sub-classes are explainedbased on the relative orientation of AGN as compare to the observer line of sight.The latest available catalog of Seyfert galaxy based on SDSS-DR7, by Zhou et al (2006),consist of about 2000 sources. The main finding based on this optical sample is that (i) Withinthe overall Seyfert 1 population, the incidence of NLSy1 galaxy is strongly dependent on theoptical, X-ray, and radio luminosities as well as the radio loudness. (ii) On average the relativeratio of FeII/H-? emission, in NLSy1 galaxy is about twice that in normal active galacticnuclei (AGN) and is anticorrelated with the broad component width of the balmer emissionlines (iii) The well-known anticorrelation between the width of broad low ionization lines andthe soft X-ray spectral slope for broad line AGN extends down to FWHM ?1000 km/s inNLSy1 galaxy, but the trend appears to reverse at still smaller line widths. (iv) The equivalentwidth of H-alpha and FeII emission lines are strongly correlated with the H-? and continuumluminosities. (v) This sample show no difference between NLSy1 galaxy and normal AGN inregard to the narrow line region. (vi) A significant correlation between M BH and ? ? is found,but with the bulk of black hole masses falling below the values expected from the M BH ? ? ?relation for normal galaxies and normal AGN. This result indicates that NLSy1 galaxies areunder age AGN, where the growth of the SMBH lags behind the formation of the galacticbulge. As a result one possibility of their small width of BLR (? 1000km/s) may be thesmaller BH-mass.4However it should be noted that the sample of 2000 NLSy1 galaxies based on SDSS-DR3is statistically very small in comparison to the large sample statistics of about 3 million AGNpopulation discover so far based on recent SDSS-DR12. Particularity large sample becomevery important while making sub-sample such a "radio loud/radio quiet", "X-ray/non-Xraydetected" and similarly the "gamma-ray detected", so as to have complete understanding ofsuch peculiar class in the context of general unification models. Therefore it is of utmostimportance to make use of the large sample of DR-12 AGN catalog, to derive the candidateNLSy1 galaxy based on optical spectra. Systematic follow up study of the un-biased statisticalsample using the multi-wavelength observation, can be accomplished in a dedicated thesis workas planned here, will be very important for understanding this less explored NLSy1 subset ofAGN.ObjectivesLarge scale sky surveys, such as Sloan Digital Sky Survey (SDSS), Baryon Oscillation Spectro-scopic Survey (BOSS) and many others have proven the power of large data sets for answeringfundamental astrophysical questions. This large volume of data available by various surveysand archive of 10m class telescope can be used in conjunction with 1-3.6m telescope followup observation to address many key problems in the research field of AGN. For instance, thequestion about the mechanism responsible for the AGN variability on diverse time scale, akey properties shown by all class of AGN, has been a matter of debate for about two decade.Now it remain for the systematics observation to rule out or confirm among various possiblemodels, the real mechanism responsible for the micro-variation seen in different class of AGN(e.g Radio-loud or Radio quiet and blazars). On the other hand, low luminosity AGN classsuch as Narrow-line Seyfert 1 (NLSy1) galaxies are thought to have very less radiation frommechanism such as relativistic jet based model. But after the launch of space telescope suchas Large Area Telescope (LAT) on-board the Fermi Gamma-ray space telescope and manyX-ray mission have detected dozen of NLSy1 galaxy in high energy band, although gamma-raydetected NLSy1 galaxies are very less in number, as only 8 are known till now. These newdetection of NLSy1 galaxy in high energy band, have open a new window for research in thefield of NLSy1 galaxy to better understand their jet and high energy mechanism.In this thesis, we aim to use the observational constrains to give insight on these aboveissues by carrying out Multi-wavelength study of these Active Galactic Nuclei, by focusing on (i)continuum monitoring of some bright NLSy1 galaxies with 1-3.6m class telescope (ii) Studying5the spectral variability and spectral analysis in multi-wavelength mostly in X-ray and tryingto find out AGN parameter such as Black hole mass (BH), bolometric luminosity, Eddingtonratio, emission line ratio etc and (iii) To carry out the X-ray, FUV and NUV ASTROSATobservation of two sub-class of NLSy1 galaxies one with significant optical variability andother with non-significant variability. Such two subsample of NLSy1 galaxies, can be selectedbased on multi-epoch SDSS spectra (about dozens epoch) over month like timescale. UsingASTROSAT multi-wavelength observation our goal will be to understand the key question suchas: Is their any significant difference in the main driving physical parameter of these two class?Like in optical, do they also show any significant difference in the UV and X-ray properties?Based on INOV study of NLSy1 galaxiesSignificant variability in brightness over a few minutes to several hours (less than a day)is commonly known as micro-variability, intranight optical variability (INOV) or intradayvariability. Optical micro-variability is a well-known property of radio-loud AGN, particularlyof its blazar subclass (e.g. Gupta et al. 2008 and references therein). The past two decadeshave witnessed a large number of INOV studies covering different classes of AGN, in orderto study the physical processes underlying this phenomenon occurring in the different AGNclasses (Miller et al. 2000, Heidt & Wagner 1996, Carini et al. 2007, Gopal-Krishna et al.1995,1993, Stalin et al. 2004b, Joshi & Chand 2013, Goyal et al. 2013, de Diego 2014).These studies have led to theoretical model for INOV ( e.g. see Ulrich et al. 1997, Czernyet al. 2008). For instance, in blazars, where pronounced INOV is observed, the cause couldbe turbulence or localized particle acceleration events within the non-thermal plasma flowingin a relativistic jet (e.g. Wagner & Witzel 1995). On the other hand, in the case of radio-quiet quasars (RQQSOs) flares occurring in the accretion disk might also play a significant ifnot dominant, role in causing the INOV (Mangalam & Wiita 1993). Recently the detectionof INOV on hour-like or shorter time scale have found in gamma-ray-loud narrow-line Seyfert1 galaxies, suggesting the presence of non-thermal jets with large doppler factors (Paliya etal. 2013). Hence, INOV studies of different classes of AGN can help to understand the AGNphysics.As a result, we would like to start a systematic programme in this thesis to make anextensive search for Intra-night optical variability (INOV) of both radio-quiet and radio loudnarrow-line Seyfert 1 (NLSy1) galaxies in order to explore much detail of their jet physics andhigh energy emission mechanism.Based on spectral properties of NLSy1 galaxiesThe origin of the X-ray emission in quasars are not well understood. In the widely accepteddisk-corona model (Haardt & Maraschi 1993), the UV soft photons from the accretion discare comptonized and up-scattered (Inverse-comptonization) into the hard X-ray band by ahot corona above the accretion disc. However, the mechanisms of energy transfer to the hotphase, and its geometry and size are not clear. Therefore, it is not well known how the basicphysical parameters (such as black hole (BH) mass, accretion rate, total luminosity) affectthe X-ray emission. Laor et al. (1997) found a correlation between the soft X-ray (0.2-2.0keV) spectral slope ? X and the FWHM of the H? emission line in a sample of 23 low-redshiftquasars, suggested that the physical parameter driving the correlation is the eddington ratio(L bol /L edd ), where L bol is the bolometric luminosity and L edd is the eddington luminosity(defined as the maximum luminosity of a source of mass M that is powered by sphericalaccretion). Further studies in the 2-10 keV energy band (e.g., Brandt et al. 1997; Shemmeret al. 2006) confirmed this correlation for the hard X-ray spectral slope, and is therefore notrelated to the active galactic nucleus (AGN) "soft excess". In particular, a ? X - L bol /L eddcorrelation has been found in a sample of ?150 Sloan Digital Sky Survey (SDSS) quasars inconjunction to their X-ray data from Chandra space telescope, spanning a large redshift range(Kelly et al. 2008). Recently, Shemmer et al. (2008) presented a similar analysis based on asample of 35 quasars, spanning more than 3 orders of magnitude in luminosity, and obtaineda stronger correlation between ? X and the eddington ratio L bol /L edd than with FWHM(H?),breaking the partial degeneracy between these two quantities. Recently Minfeng Gu (2009)have found the anticorrelation between the hard X-ray photon index (? X ) and the eddingtonratio (L bol /L edd ) in low-luminosity active galactic nuclei (LLAGN).Nebular emission lines are a powerful tool for diagnosing the physical state of ionized gasand studying central nuclear activity. Optical emission line ratios can be used to discriminatebetween emission from the star formation in galaxies and harder radiation such as from thecentral nuclear activity around a supermassive black holes (e.g., Baldwin et al. 1981; Veilleux& Osterbrock 1987; Kewley et al. 2001; Kauffmann et al. 2003). Compared to star form-ing galaxies, AGN produce greater numbers of higher energy photons (e.g., UV and X- rays)and, therefore drive higher ratios of the collisionally excited forbidden lines compared to thephoto-ionization-induced balmer emission lines. Although such line ratios provide useful AGNdiagnostics, even for obscured AGN (Reyes et al. 2008; Yuan et al. 2016). Kyuseok oh etal.(2016) have explore this aspect in much detail for 642 optical spectra for non-beamed AGNfrom the BAT AGN Spectroscopic Survey Data Release 1 (BASS DR1) and found a significant7correlation between line ratio N II ?6583 /H ? and eddington ratio L bol /L edd . The main limita-tion of past studies involving X-ray slopes is that either they rely on low-quality X-ray data,or they are based on relatively small samples. The availability of large, homogeneous samplesof AGN with high-quality optical and X-ray spectral data may now open new possibilities inthis field.In this thesis we would like to work towards these goals by exploiting the X-ray spectraavailable from ROSAT, XMM-Newton X-ray telescope and optical spectra available from SloanDigital Sky Survey (SDSS), as well as by using 2-3.6m class telescope in India (e.g IGO, HCTand DOT) for NLSy1 galaxies for large sample so as to make the correlation between X-rayphoton index (? X ) and Nebular emission lines with eddington ratio (L bol /L edd ) statisticallysignificant.Probing narrow-line Seyfert 1 (NLSy1) galaxies with and without sig-nificant variation in optical wavebandWith the recent large survey such as SDSS-DR12, not only the sample size of NLSy1 galaxieshave increased but one can also study the variation in spectral properties from their multi-epochSDSS-spectrum. Such large sample has resulted in two class of NLSy1 galaxy with and withoutsignificant variation in their spectral properties over month like time scales. This also give riseto interesting question such as, is their any significant difference in the main driving physicalparameter of these two class? Like in optical, do they also show any significant difference inthe UV and X-ray properties? The spectral energy distribution (SED) based on simultaneouslyobservation in multi-wavelength, using ASTROSAT along with simultaneous optical coverageusing ARIES 3.6m DOT will be highly rewarding to understand these two possible sub-classesof NLSy1 galaxies with and without significant variation in their optical properties.Noteworthy contribution in the field of proposed workWith our above proposed objectives we can make following noteworthy contribution in theresearch field of AGN.• Based on INOV study of NLSy1 galaxies: Since there are only very less numberof gamma-ray detected NLSy1 galaxies known and out of 8 known gamma-ray detectedNLSy1 galaxies till now only around 4 have been monitored for INOV study. With ourplaned INOV programme we wish to observe all gamma-ray detected NLSy1 galaxiesknown till now. In addition to this we wish to do INOV of bright X-ray detected source8having detection in X-ray more than 3? level but not detected in gamma-ray, to comparetheir INOV nature with gamma-ray detected NLSy1 galaxies and blazar class of AGN.The outcome will be very important to know whether gamma-ray detected or X-raydetected NLSy1 galaxies INOV characteristics properties similar or different than thatof blazar class of AGN whose emission are dominated by jet.• Spectral properties of NLSy1 galaxies: For the first time we have proposed to in-vestigate systematically the correlation between X-ray photon index (? X ) and eddingtonratio L bol /L edd and nebular emission line ratio with eddington ratio L bol /L edd for thelarge sample of NLSy1 galaxies, to understand the origin of X-ray emission in muchdetail. These correlation will be helpful to shed important light on the key parametergoverning the activity of AGN central engine.• Probing narrow-line Seyfert 1 (NLSy1) galaxies with and without significantvariation in optical waveband: Using SDSS-DR12, not only the sample size of NLSy1galaxies have increased but one can also study the variation in spectral properties fromtheir multi-epoch SDSS-spectrum. Such large sample has resulted in two class of NLSy1galaxy with and without significant variation in their spectral properties over monthlike time scales. Using ASTROSAT multi-wavelength observation of such two subset ofNLSy1 galaxy, in this thesis work we will be able to understand the key puzzle such as:Is their any significant difference in the main driving physical parameter of these twoclass? Like in optical, do they also show any significant difference in the UV and X-rayproperties?Proposed methodologyOur groups have long experience of handling photometer and imaging data especially us-ing 1.3m ARIES telescope.Also spectral data reduction (both optical and X-rays), ourgroup has long working experience of high/low resolution spectroscopic data of UVES/VLT,HARPS/ESO, EFOSC/ESO, HCT, XXM-newton and ROSAT. In brief the following method-ology will be adopted:Continuum variabilityFor the Continuum variability study, we will be using :• 1.3m-Devastal Fast Optical Telescope(DFOT), ARIES.• 2m-Himalayan Chandra Telescope(HCT).9• 2m-IUCAA Girawali Observatory(IGO).• 3.6m-Devasthal Optical Telescope(DOT), ARIESwhich will be ideal for our science goal.Spectral propertiesThe spectral properties of AGN will be studied using data from archive as well as 2m-classtelescope facilities in INDIA e.g.• 2m-Himalayan Chandra Telescope(HCT).• 2m-IUCAA Girawali Observatory(IGO).• 3.6m-Devasthal Optical Telescope(DOT), ARIES.• ASTROSAT : Multi-wavelength data.Archive Data:• Optical spectra from Sloan Digital Sky Survey (SDSS).• X-ray spectra from XMM-Newton & ROSAT space telescope archive.• High resolution spectra from UVES/VLT, HARPS/ESO, Keck Telescope archive.Expected outcome of the proposed workWith our above proposed objectives we expect following outcome:• Based on INOV study of NLSy1 galaxies: Since there are only very less numberof gamma-ray detected NLSy1 galaxies known and out of 8 known gamma-ray detectedNLSy1 galaxies till now only around 4 have been monitored for INOV study. With ourplaned INOV program we wish to observe all gamma-ray NLSy1 galaxies known till now.In addition to this we wish to do INOV of bright X-ray detected source having detectionin X-ray more than 3? level, to compare their INOV nature with gamma-ray NLSy1galaxies and blazar class of AGN. With the observation of these two class of NLSy1galaxy (i) Gamma-ray detected NLSy1 galaxy and (ii) X-ray detected but not gamma-ray detected NLSy1 galaxy, we will for the first time be able to acertain the gamma-ray10detected NLSy1 galaxy duty cycle is similar or comparable to as X-ray detected butnot gamma-ray detected NLSy1 galaxy. We also note here that if gamma-ray detectedNLSy1 galaxy do really show a substantially higher duty cycle for micro-variability thanX-ray detected but not gamma-ray detected NLSy1 galaxy, this would shed light on thequestion of whether or not gamma-ray detected NLSy1 galaxies are special cases of theNLSy1 galaxy, especially in terms of their micro-variability properties. In addition tothis we will also be able to compare the INOV characteristics of gamma-ray detectedNLSy1 galaxy with blazar class of AGN whose emission are dominated by jet.• Spectral properties of NLSy1 galaxies: For the first time we have proposed toinvestigate systematically the correlation between X-ray photon index (? X ) and edding-ton ratio L bol /L edd and nebular emission line ratio with eddington ratio L bol /L edd forthe large sample of NLSy1 galaxies, to understand the origin of X-ray emission. Withthese correlation we will be able to validate the well known disk-corona model (Haardt& Maraschi 1993) of X-ray emission, where UV soft photons from the accretion discare comptonized and up-scattered (Inverse-comptonization) into the hard X-ray band bya hot corona above the accretion disc or advection-dominated accretion flow (ADAF)model.• Probing narrow-line Seyfert 1 (NLSy1) galaxies with and without significantvariation in optical waveband: To understand the physical parameter governing thedifference in these two sub-classes one belong to with significant variability and anotherwith non-significant variability, spectral energy distribution (SED) based on simultaneousmulti-waveband observation with ASTROSAT will be very rewarding, as their BH massare accurately determined from SDSS RM and optical follow-up will be done with ARIES3.6m Devasthal Optical telescope. Further, any difference if found in the X-ray and UVSED of these two sources will also give important clue about the presence/absence ofwarm absorber in these two classes for further investigation with larger statistical sample.In the initial part of this long term programme, two of sources from our sample has beenalready observed for more than 27hr by ASTROSAT in multi-band.